After treating mammalian embryos during late gestation with various chemicals, a correlative study will be made to determine precise cellular deficits in various areas of the brain and behavioral abnormalities later in life. Studies on repair in the central nervous system will be continued, by treating mammalian embryos at specific days of development and by studying the compensatory mechanisms, particularly in the spinal cord. A study is presently undertaken to determine whether proliferative cell populations in the central nervous system are more sensitive to DNA-inhibiting substances than proliferating populations in other organs. An electronmicroscopy study is planned to examine the interkinetic movements and migration of neurons after treatment with various chemicals.